CSIRO LAND AND WATER
Relating canopy cover and average height to the biomass of the stand
John Larmour, Micah Davies, Keryn Paul, Jacqui England, Stephen Roxburgh
August 2018
Report prepared for the Department of the Environment and Energy
Citation
Larmour, J, Davies, M, Paul, K, England, J, Roxburgh, S, (2018). Relating canopy cover and average height to
the biomass of the stand. Report prepared for the Department of the Environment and Energy. CSIRO Land
and Water, Canberra.
Copyright
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Contents
Acknowledgments ...................................................................................................................... ii
1 Background .................................................................................................................... 4
1.1 Objectives ......................................................................................................... 4
2 Methods ......................................................................................................................... 5
2.1 Individual-level measurements of shrub and tree stem diameter and crown
area 5
2.2 Individual-level measurements of shrub and tree stem diameter and height 7
2.3 Stand-level estimates of biomass, canopy cover and height ........................... 7
3 Results and Discussion ................................................................................................. 10
3.1 Relationship between stem diameter and crown area of individuals ........... 10
3.2 Relationship between stem diameter and height of individuals ................... 10
3.3 Relationship between stand-level above-ground biomass and canopy
cover 11
3.4 Relationship between stand-level above-ground biomass and average
height 12
3.5 Comparison of observed versus predicted canopy cover and height ............ 13
4 Conclusions .................................................................................................................. 15
5 References ................................................................................................................... 16
ii
Acknowledgments
The project was funded by Department of the Environment and Energy. We thank Keiran Andrusko for
assistance with site selection and Glenn Newnham for his review of an earlier draft.
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1 Background
1.1 Objectives
The aim of this study was to explore the relationship between the above-ground biomass of a stand of woody vegetation and the canopy cover in semi-arid regions of Australia. The specific objective was to develop individual tree- or shrub-based relationships between stem diameter and crown area that could then be applied to plot-based inventories of stem diameter to derive crown areas and scale-up to stand-based estimates of crown cover. The focus of the new field work was on providing additional datasets to build confidence that the relationships derived using existing datasets are more broadly applicable.
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2 Methods
2.1 Individual-level measurements of shrub and tree stem diameter and crown area
We identified existing datasets of 1,677 individual shrubs, multi-stemmed trees or single-stemmed trees where stem diameter and crown area were measured in semi-arid regions of Western Australia (WA), New South Wales (NSW) and Queensland (Qld) (Table 1). Just over half of these measurements (59%) were taken especially for this study from locations near Bourke and Charleville (Fig. 1).
Table 1: Type and number of shrubs and trees that were measured for both stem diameter and crown area within semi-arid regions of Australia.
Plant Functional Type & Genus Region N
Shrubs
Atriplex spp. Near Canna, Central West, WA 54
Rhagodia spp. Near Canna, Central West, WA 52
Melaleuca spp. Near Canna, Central West, WA 47
Acacia tree spp.2 Near Canna, Central West, WA 124
Melaleuca spp. Near Ravensthorpe, South-West WA 40
Dodonaea spp. Near Cobar, W NSW
46
Eremophila sturtii (turpentine bush) Near Cobar, W NSW 50
Dodonaea viscosa SW of Bourke, NSW 195
Eremophila sturtii SW of Bourke, NSW 106
Multi
Acacia shrub spp.1 Near Canna, Central West, WA 124
Eucalyptus horistes (mallee) Near Canna, Central West, WA 152
Acacia aneura (mulga) Near Bourke, W NSW 198
Acacia aneura (mulga) Near Charleville, Qld 54
Trees
Allocasuarina cristata Near Bourke, W NSW 47
Flindersia maculosa Near Bourke, W NSW 27
Eucalyptus populnea (poplar box) Near Bourke, W NSW 39
Corymbia intermedia Near Charleville, Qld 42
Corymbia tessellaris Near Charleville, Qld 58
Callitris glaucophylla Near Charleville, Qld 57
Eucalyptus melanoploia Near Charleville, Qld 47
Eucalyptus coolabah Near Charleville, Qld 57
Eucalyptus populnea (poplar box) Near Charleville, Qld 61
Total 1,677 1A. acuminata, A. saligna 2A. hemiteles, A. murrayana, A. victoriae
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Figure 1: Examples of some of the individuals measured for stem diameter and crown area: a) & b) shrub-dominated
plots measured at Bourke; c) & d) tree-form species from Bourke region.
For all 1,677 individuals measured, crown area was calculated as pi times half the crown length (diameter) measured in an East-West orientation, and times half the crown length measured in a North-South orientation. The stem diameter was measured at 10 cm height above the ground (D10). Given all of the shrubs and trees measured were from semi-arid regions of Australia, most of the individuals measured were multi-stemmed (Fig. 2). All measurements of multi-stemmed individuals (Di) were converted to a single value (equivalent stem diameter, De = √∑Di
2, cm), such that the total basal area (cm2) for all stems was equal to the basal area of an individual with this equivalent single diameter.
Figure 2: Examples of multi-stemmed individuals measured for stem diameter.
a) b)
c) d)
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These datasets were used to determine if there was a generic relationship between D10 and crown area across all datasets, as well as across only datasets where D10 < 30 cm given this subset of data better represented smaller regenerating individuals. We also used ANCOVA analysis to assess whether this relationship was dependent on the plant functional type; namely whether the individual was a shrub, multi-stemmed tree, or single-stemmed tree.
2.2 Individual-level measurements of shrub and tree stem diameter and height
Empirical relationships that relate height to stem diameter of key plant functional types in Australia have been reported previously by Paul et al. (2016; see Table S2 of Supplementary Material). Here we used the same dataset of Paul et al. (2016) and derived relationships for shrubs, multi-stemmed trees (namely multi-stemmed acacias and mallees) and single-stemmed trees excluding relatively large (D10 > 30 cm) individuals from the analysis. For all plant functional types, heights were related to the stem diameter measured at 10 cm above the ground. A further 738 and 142 individuals with D10 >30 cm were also included in the dataset from the recent survey at Bourke and Charleville, respectively. Therefore the total dataset included 2,800 shrubs, 3,493 multi-stemmed trees, and 2,994 single-stemmed trees. These datasets were used to determine if there was a generic relationship between D10 and height of an individual shrub, multi-stemmed tree or single-stemmed tree.
2.3 Stand-level estimates of biomass, canopy cover and height
Plot-based surveys of woody biomass traditionally use plot- or transect-based inventories of stem diameter. These inventory datasets are then used to estimate woody biomass through the application of allometric relationships for above-ground biomass (Paul et al. 2016) and below-ground biomass (Paul et al. 2018). Using the diameter-based relationship for prediction of crown area (Section 2.1) or height (Section 2.2), these same inventories may also be utilised to provide corresponding estimates of stand-based canopy cover and stand-based estimates of average height of the woody vegetation.
Here we collated plot-based inventories of stem diameter from 790 existing datasets, the primary focus being semi-arid regions of Australia (Table 2). To be included in the data analysis the inventory datasets were required to meet the following criteria:
1. Estimated total (above-ground and below-ground) biomass carbon, TBC <10 t C ha-1; 2. Plot size > 0.04 ha, and; 3. >10 individuals measured per plot.
As outlined in Table 2, there were three types of stands assessed:
1. Natural regeneration, where remnant trees within the measured plots were excluded from the analysis (Fig. 3);
2. Environmental plantings, where remnant trees within the measured plots were excluded from the analysis (Fig. 4), and;
3. Other native vegetation (Fig. 5).
The latter group generally included sites where the management history was not well known, and/or where
it was difficult to distinguish the remnant vegetation from that which has been recently regenerated. A total
of 14 new plots were measured in the Bourke region for individual canopy areas, heights and stem diameters
of trees/shrubs. This region of western NSW typically has large areas dominated by shrubs (hop and
turpentine bush) with limited natural regeneration of trees, typically as a consequence of competition from
shrubs and high levels of grazing pressure from feral goats. Based on the types of stands available, it was
necessary to modify the plot measurement methodology in order to measure enough individual trees for
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crown area and height. Thus only five of the 14 plots were included in the dataset for stand-level estimates;
the other plots did not meet the criteria above of having a TBC < 10 t C ha-1.
Table 2: Number of stem-diameter inventory plots (N) collated from stands under regeneration, environmental plantings or other native vegetation in semi-arid areas of Australia. NBL refers to the National Biomass Library.
Type of stand Source N
Natural regeneration
Canegrass and Wallal sites, near Charleville CSIRO 209
Curra and Meadows sites, near Cobar CSIRO 16
Conservation areas, near Bourke CSIRO 5
Gidgee sites NBL 2
Mulga sites NBL 108
Environmental plantings
22 sites, SE Australia CSIRO 47
9 sites, Central West, WA CSIRO 326
Other native vegetation
9 studies NBL 63
9 studies CSIRO 103
Total 879
The datasets in Table 2 were used to explore whether there was a relationship between stand-average height and stand-average TBC, and whether this relationship differed between the three types of stands. Similarly, the collated datasets were used to explore whether there was a relationship between stand-average woody canopy cover and the stand-average TBC, and whether this relationship differed between stand types.
Stand-average TBC (t C ha-1) was calculated as the sum of the above- and below-ground biomass of all individuals in the plot, multiplied by 0.5, and divided by the area of the plot. Stand-average woody canopy cover (m2 m-2) was calculated by summing the crown areas of all individuals within the plots and dividing this total crown area by the area of the plot.
The 14 plots measured at Bourke were the only stands where both measured and predicted canopy cover and average height were available. Therefore these plots provided a test of how well predictions of canopy cover and average height matched that observed.
Figure 3: Example of natural regeneration stands where inventories of stem diameter were undertaken.
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Figure 4: Example of environmental planting stands where inventories of stem diameter were undertaken.
Figure 5: Example of ‘other native vegetation’ where inventories of stem diameter were undertaken.
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3 Results and Discussion
3.1 Relationship between stem diameter and crown area of individuals
The empirical relationship between stem diameter (D10) and crown area of individual shrubs and trees (see Section 2.1) was best described by a power relationship (Canopy area = 0.0807D10
1.7626), with an R2 of 0.867 and RMSE of 19.9 (N = 1,677) (Fig. 6). This relationship appeared to be generic, with no indication that it differed between shrubs and trees. However for this study, our interest was mostly in the smaller regenerating individuals with D10 < 30 cm. For these individuals, the best power relationship describing this smaller subset of data (Canopy area = 0.1699D10
1.5161; N = 1,387) gave and R2 of 0.663 and RMSE of 4.9 (Fig. 6).
Figure 6: Relationship between stem diameter and the canopy area of an individual shrub or tree (N = 1,677). Datasets used are given in Table 1.
3.2 Relationship between stem diameter and height of individuals
The individual shrub and tree datasets compiled by Paul et al. (2016) and the additional data from Bourke were used to determine an empirical relationship between stem diameter (D10) and height as described in Section 2.2. A power relationship could be fitted to the dataset, but with the relationship clearly differing between trees and shrubs (Fig. 7). The power relationship for single-stemmed trees had an R2 of 0.7496 (N =
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Stem diameter at 10 cm height (cm)
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2,994), for multi-stemmed trees had an R2 of 0.5916 (N = 3,493), while for shrubs the relationship had an R2 of 0.6862 (N = 2,800).
Figure 7: Relationship between stem diameter and the height of an individual shrub, single- or multi-stemmed tree. Datasets used are from Paul et al. (2016) and the new data from Bourke, and excluded shrubs and trees of relatively large size (D10 >30 cm).
3.3 Relationship between stand-level above-ground biomass and canopy cover
Although there was large variation between plots, there was nevertheless a clear relationship between TBC and the crown cover of a stand. A power relationship could be fitted to these datasets, with an R2 of 0.8269 (N = 892) (Fig. 8). This relationship appeared to be generic, with no clear indication that it differed between the different types of stands. However, there was a large amount of prediction error for ‘Other native vegetation’, probably due to variation in remnant trees included within their inventory assessments. For environmental plantings, these TBC-crown relationships are anticipated to vary largely based on the configuration in which the shrubs and trees are established, e.g. rows with wide inter-row spaces cf. dense block plantings. Of most interest is the TBC-crown relationship for stands of natural regeneration. For datasets from stands of natural regeneration, the power relationship could be fitted with an R2 of 0.7865 (N = 349) (Fig. 8).
y = 0.8568x0.7972
R² = 0.7496
y = 0.7895x0.747
R² = 0.5916
y = 0.8228x0.5568
R² = 0.6862
0
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Stem diameter at 10 cm height (cm)
Single-stemmed tree
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Power (Single-stemmed tree)
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Power (Shrub)
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Figure 8: Relationship between the above- and below-ground total biomass carbon (TBC) and woody canopy cover of a stand (m2 m-2) in moderate-low rainfall and semi-arid regions of Australia. Datasets used are given in Table 2.
3.4 Relationship between stand-level above-ground biomass and average height
The stand-level relationship between TBC and average height was relatively poor (Fig. 9). This was particularly the case for ‘Other native vegetation’ where not all remnant trees were excluded from the inventory assessments given the difficulty in distinguishing remnant trees from those regenerating. Indeed a number of ‘Other native vegetation’ stands with TBC of 4 to 7 t C ha-1 probably contained some remnant trees given their relatively high heights (> 5 m). These same stands also contributed to the relatively high TBC for a given canopy cover of only about 0.05-0.08. See sites circles in red in Fig. 8 and 9.
For regenerating stands, the fitted power relationship had an R2 of only 0.1596 (Fig. 9), with typical heights of regenerating stands being 1.0 to 5.5 m. However, it appeared that there were still a proportion of regenerating stands that did not attain an average height of 2 m at the 3.85 t C ha-1 of TBC required to reach a canopy cover of 0.2 (Fig. 8).
y = 0.0733x0.7008
R² = 0.8269
y = 0.0719x0.7592
R² = 0.7865
0.0
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All data
Environmental Plantings - Remnants excluded
Other native vegetation- May have some remnants
Regeneration - Remnants excluded
Power (All data)
Power (Regeneration - Remnants excluded)
All data
Regeneration only
NBL 'Other native vegetation' plotsthat include trees at very low stand densities(average 75 trees per hectare; range 22-166)
3.854.20
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Figure 9: Relationship between the above-ground biomass (AGB) and average height of a stand in moderate-low rainfall and semi-arid regions of Australia. Datasets used are given in Table 2.
3.5 Comparison of observed versus predicted canopy cover and height
The 14 plots measured at Bourke were the only stands where both measured and predicted canopy cover and average height were available. Therefore these plots provided a test of how well predictions of canopy cover and average height matched that observed. Our results showed that when applying the power relationship shown in Fig. 6, there was negligible bias in prediction of canopy area of individual trees and shrubs measured (Fig. 10a), and hence, there was also negligible bias in the plot-based estimates of canopy cover (Fig. 10c). Similarly, when applying the power relationships shown in Fig. 7, there was negligible bias in prediction of the height of individual trees and shrubs measured (Fig. 10b), and hence, there was also negligible bias in the plot-based estimates of average stand height (Fig. 10d).
y = 2.1442x0.2347
R² = 0.2721
y = 2.2092x0.1817
R² = 0.1596
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All data
Environmental Plantings- Remnants excluded
Other native vegetation- May have some remnants
Regeneration- Remnants excluded
Power (All data)
Power (Regeneration- Remnants excluded)
Regeneration only
All data
Same NBL 'Other native vegetation' plotsthat include trees at very low stand densities(average 75 trees per hectare; range 22-166)
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Figure 10: For the study sites near Bourke, a comparison between observed and predicted: (a) canopy area of individual trees and shrubs, when applying the power relationship fitted to data shown in Fig. 6; (b) height of individual trees and shrubs measured, when applying the power relationships fitted to data shown in Fig. 7; (c) plot-average canopy cover at the 14 plots measured, when applying the power relationship fitted to data shown in Fig. 6 and accounting for the area of the plots, and (d) plot-average height at the 14 plots measured, when applying the power relationships fitted to data shown in Fig. 7 and accounting for the area of the plots.
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4 Conclusions
The relationship between stem diameter and crown area of individual trees and shrubs appears to be robust
(R2 = 0.83) for the semi-arid woodland and shrubland regions included in this study. Further work is required
to test if this relationship holds for different climatic regions. A reasonable relationship between stand-level
estimates of canopy cover and above-ground biomass was found, particularly for natural regeneration,
although this varied according to the extent to which stands included remnant trees. If this relationship can
be validated more broadly, crown areas could be utilised to estimate biomass from aerial surveys, either
from detailed plot-based drone surveys or from broad-scale airborne or satellite imagery. Crown areas shown
on current imagery and compared to historic images, could be an effective method to estimate changes in
biomass over varying time scales. We suggest additional tree crown area studies in a broader range of
woodland systems, such as those in the southern tablelands of NSW and south western Queensland, to
further test the relationship between crown area and stem diameter.
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5 References
Paul, K.I., Roxburgh, S.H., Chave, J. England, J., Zerihun, A., Specht, A., Lewis, T., Bennett, L.T., Baker, T.G., Adams, M.A., Huxtable, D., Montagu, K.D., Falster, D.S., Feller, M., Sochacki, S., Ritson, R., Bastin, G., Bartle, J., Wildy, D., Hobbs, T., Larmour, J., Waterworth, R., Stewart, H.T.L., Jonson, J., Forrester, D.I., Applegate, G., Mendham, D., Bradford, M., O’Grady, A., Green, D., Sudmeyer, R., Rance, RJ., Turner, J., Barton, C., Wenk, E.H., Grove, T., Attiwill, P.M., Pinkard, E., Butler, D., Brooksbank, K., Spencer, B., Snowdon, P., O’Brien, N., Battaglia, M., Cameron, D.M., Hamilton, S., McArthur, G., Sinclair, J. (2016). Testing the generality of above-ground biomass allometry across plant functional types at the continent scale. Global Change Biology, 22, 2106-2124.
Paul, K.I., Larmour, J., Specht, A., Zerihun, A., Ritson, P., Roxburgh, S.H., Sochacki, S., Lewis, T., Barton, C., England, J., Battaglia, M., O’Grady, A., Pinkard, E., Applegate, G., Jonson, J., Brooksbank, K., Sudmeyer, R., Wildy, D., Montagu, K., Bradford, M., Butler, D., Hobbs, T. (2018). Testing the generality of below-ground biomass allometry across plant functional types at the continental scale. Forest Ecology and Management. In press.
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